Method of and an apparatus for projecting a scanned two- or three-dimensional modulated light pattern originating from a light source

ABSTRACT

A scanned 2- or 3-D modulated light pattern originating from a light source and represented by a matrix of pixels is projected onto a defined surface or into a defined region distant from the light source. A plurality of means for modulating light one operated between at least a first mode and a second mode. This operation is synchronized with the scanning of the matrix of pixels so that: 
     (a) respective modulating means is in the first mode when its associated plurality of pixels are being scanned; and (b) a respective plurality of pixels are not being scanned when its associated modulating means is in the second mode. When a modulating means is in the first mode, it allows light to be distributed from the defined surface or the defined region. When a modulating means is in the second mode, it allows the effect produced by any light incident on the desired surface or in the defined region to be reduced.

The present invention relates to a method of and an apparatus forprojecting a scanned two- or three-dimensional modulated light patternoriginating from a light source. Such a light pattern could be an imagefrom a CRT video projector. In particular, this invention relates tosuch a method and an apparatus in which the effect of ambient light isreduced.

GB 1604375 (British Aerospace) discloses an electro-optical scanningapparatus for converting between a picture and an electrical picturesignal of which respective portions are representative of respectiveelements of the picture. The apparatus disclosed is primarily for use inscanning an image but can be modified to produce an image. In themodification, the apparatus comprises a linear array of light emittingdiodes (LEDs), each of the diodes being individually addressable, and aliquid crystal device. In operation, one line of the liquid crystaldevice is activated, becoming transparent, then as each of the LEDs isaddressed in sequence along the length of the array an image of theemitted light is formed on that line of the liquid crystal. This line isthen deactivated and the next is addressed and so on until the entireimage has been formed. The entire image is produced by the interactionof the LEDs which emit the light and the activation and deactivation ofthe lines of the liquid crystal device which act as an electro-opticalshutter to produce the vertical scanning.

EP 0422 (Hunt) discloses an optical screen for use in particular withcathode ray tubes of the type in which an electron beam is rapidly movedover a large number of parallel lines on a display face to produce animage on that display face. The optical screen includes a number ofstrips aligned parallel to the direction of writing motion, each stripincluding an electro-optical substance. A strip immediately forward ofthe electron beam is transparent whilst the other strips are absorbentand non-reflecting. As the electron beam moves down the screen and thelight emission behind the transparent strip fades, this strip can beswitched to absorbent and non-reflecting at a convenient time. Such anoptical screen enhances the contrast of the image produce on the displayface. However, there is no disclosure of the use of such a system toproduce a projected image.

It is an object of the present invention to provide a method of and anapparatus for projecting a scanned two- or three-dimensional modulatedlight pattern originating from a light source.

According to a first aspect of the present invention there is provided amethod of projecting a scanned two- or three-dimensional modulated lightpattern originating from a light source and represented by a matrix ofpixels, the method comprising the steps of:

projecting said pattern onto a defined surface or into a defined regiondistant from the light source;

operating a plurality of means for modulating light between at least afirst mode and a second mode, each of the modulating means beingassociated with a plurality of pixels; wherein the scanning of thematrix of pixels and the operation of said plurality of modulating meansare synchronised so that (a) a respective said modulating means is insaid first mode when its associated plurality of pixels are beingscanned and (b) a respective plurality of pixels are not being scannedwhen its associated said modulating means is in said second mode;

and each of said modulating means in said first mode allows light to bedistributed from said defined surface and each of said modulating meansin said second mode allows the effect produced by any light incident onsaid defined surface or in said defined region to be reduced.

By this method, a projected light pattern resulting in an image with anenhanced contrast is produced.

According to the second aspect of the present invention there isprovided an apparatus for projecting a scanned two- or three-dimensionalmodulated light pattern originating from a light source and representedby a matrix of pixels, the apparatus comprising a light source;

means for projecting a scanned two- or three-dimensional modulated lightpattern from said light source onto a defined surface or into a definedregion distant from the light source;

a plurality of means for modulating light having at least a first modeand a second mode, each of the modulating means being associated with aplurality of pixels;

means for operating each of said modulating means between said first andsaid second mode;

and means for synchronizing the scanning of the matrix of pixels and theoperation of said plurality of modulating means so that, in use, (a) arespective said modulating means is in said first mode when itsassociated plurality of pixels are being scanned and (b) a respectiveplurality of pixels are not being scanned when its associated saidmodulating means is in said second mode;

wherein each of said modulating means in said first mode allows light tobe distributed from said defined surface and each of said modulatingmeans in said second mode allows the effect produced by any lightincident on said defined surface or in said defined region to bereduced.

Such an apparatus can be used to implement the method provided inaccordance with the first aspect of the present invention.

Embodiments of the present invention will now be described, by way ofexample only, and with reference to the accompanying schematic drawingsin which:

FIG. 1 shows a first method of producing a projected image;

FIG. 2 shows a modulating means used in the method of FIG. 1;

FIG. 3a shows a directional reflector which may be used in the method ofFIG. 1;

FIG. 3b is a polar diagram;

FIGS. 4 and 5 show a second and a third method of producing a projectedimage;

FIG. 6 shows a modulating means used in the method of FIG. 5;

FIGS. 7 and 8 illustrate variations to the methods of FIGS. 1, 4 and 5;

FIG. 9 shows a method of producing a projected image using a cathode raytube raster scan;

FIG. 10 shows a modulating means used in the method of FIG. 9;

FIG. 11 shows a modification of the method of FIG. 4 to produce aback-illuminated screen.

FIGS. 12a, 12b and 12c shows a method of producing a projected imagesuperimposed on printed matter;

FIGS. 13 and 14 illustrate the use of the method to provide a `one-way`window.

A first embodiment of a first method of enhancing the contrast betweenan image projected by a first source of light onto a defined surface,i.e. a screen, and the noise produced by ambient light from a secondsource of light is illustrated in FIG. 1. A source picture 2, such as aconventional LCD (e.g. an active matrix, a multiplexed twisted nematicor a ferroelectric colour TV LCD), a photographic transparency, or amovie film frame, is projected from a projector 3 using a Xenon strobelamp 4, or another source of light whose intensity can be varied. Theprojector is otherwise of a conventional type or may be a lenslessshadow projector, e.g. of the Linnebach type (i.e. using a `point`source to project an enlarged colour shadow image onto a screen).

The lamp 4 is flashed at a given frequency, typically of 50 Hz with eachflash lasting for an interval in the range of from of the order of 1microsecond to a few milliseconds. In order to produce an image whichappears continuous, i.e. with no flicker, the given frequency has to beequal to or greater than a critical fusion frequency, which is affectedby both the luminance and the size of the image. The luminance of acontinuous image so produced is equal to the average luminance of theflashing image.

The light from the flashing lamp 4 is projected via an opticalcollimation system 6 and a projection lens 8 and focussed onto a planeP₁ in which is situated means 10 for modulating light. The modulatingmeans 10 provides the defined surface and is operable between a firstmode and a second mode, operation being synchronised with the variationof light intensity so that more light is emitted to project the imageonto the plane when the modulating means 10 is in the first mode.

The modulating means 10 comprises an electro-optic layer containing aliquid crystal, e.g. a cholesteric to nematic phase change cell, orpreferably, as shown in FIG. 2, a sheet 12 of plastics material whichcontains microencapsulated droplets of a nematic liquid crystal material(manufactured, inter alia, under the name NCAP by Taliq Corporation,Sunnyvale, Calif., U.S.A.). On either side of the sheet 8 is laminated aplastic foil 14, 16 with transparent indium tin oxide electrodes on itssurface. This forms a large area, e.g. 1 m×3 m, electro-optic shutterwhich goes from translucent to transparent when a potential differenceis applied. Accordingly, the material can be rapidly switched (with aresponse time of about 1 ms to 5 ms) electrically between a first modein which the modulating means 10 is translucent and a second mode inwhich the modulating means is transparent.

The modulating means 10 in the first mode forms a translucent screenfrom which an image 9 (i.e. the required visual signal) is reflected tobe directly observed by an observer (indicated at 17) on the same sideof the modulating means as the projector 3 (i.e. front-projection). Inthe second mode, the modulating means 10 is transparent and so no orlittle light can be diffusely reflected from the plane P₁ to theobserver. In this way, the effect produced from ambient light incidenton the defined surface of the modulating means is reduced. One or morelayers 18, 20 are provided behind the modulating means 10 to preventambient light which reaches behind the modulating means 10 beingobserved by the observer 17 when the modulating means is transparent. Ablack material would be suitable for one of the layers 18 to absorbambient light incident thereon which reaches behind the modulating means10. A louvred sheet 20 can be used behind the modulating means 10,further to prevent ambient light which reaches behind the modulatingmeans being seen by the observer. In a preferred embodiment, the layer18 is a front-silvered mirror and is used in conjunction with thelouvred sheet 20. The front-silvered mirror 18 would reflect, say, theimage of a dark wall when the modulating means is in the second mode andthe mainly forward scattered light of the image when the modulatingmeans is in the first mode to give a brighter reflected image.

Alternatively a directional reflector can be used which reflects lightincident thereon within one or more predetermined ranges of angles butabsorbs light incident thereon outside the one or more predeterminedranges of angles. In such a case, the projector is positioned to projectlight onto the modulating means 10 and the directional reflector atangles within the one or more predetermined ranges of angles while theobserver 17 is positioned to view the modulating means 10 outside theone or more predetermined ranges of angles. Thus when the modulatingmeans 10 is in the first (scattering) mode, the image reflected from themodulating means 10 is enhanced by light from the directional reflectorwhich can be scattered by the modulating means 10 towards the observer17 whereas when the modulating means 10 is in the second (transparent)mode, light incident on the directional reflector is either absorbed orreflected away from the observer 17. One type of directional reflectoris disclosed in U.S. Pat. No. 4,726,662 (Cromack). Another type ofdirectional reflector 21 is shown in FIG. 3a. The reflector 21 comprisesa sheet of plastics material 22, one side of which is embossed with aseries of uniform parallel prisms, and an absorbing layer 23 extendingalong and separated from the other surface of the prismatic sheet 22. Onthe embossed side of the prismatic sheet, every other face has both areflective layer 24a and an absorbing layer 24b coated thereon. Asindicated by the light rays shown and the polar diagram of reflectivity(axis 23' indicating the surface 23) light incident on the reflector 21within a predetermined range of angles is reflected whereas lightincident outside the predetermined range is absorbed.

FIG. 1 also shows an anti-reflection film 25 in front of the modulatingmeans to reduce reflection of ambient light incident on the frontsurfaces of the modulating means 10. Thus the flashing image ismaximally directed to the eye of the observer 17 when the modulatingmeans 10 is in the first mode, whereas any ambient light is attenuatedduring intervals between the flashes. Accordingly, the contrast betweenthe projected image, i.e. the required visual signal, and the noiseproduced by scattered ambient light from a second source of light isimproved.

Thus, a projected image of the same contrast as prior art projectedimages can be produced by the method of the present invention using alight source of lower average power, or alternatively a projected imageof better contrast can be produced using a light source of the sameaverage power.

FIG. 1 also shows schematically the electrical circuitry used in themethod. Waveforms generated from a source 26, optionally modified bymeans 28, control a pulse generator 30. The source 26 could be a 50 HzAC mains supply or the field synchronisation pulses from a television,e.g. an LCD TV at 2, with the generated waveforms being modified bymeans 28 such as a full-wave rectifier and comparator. Alternatively,the source 26 could be a photodetector which measures pulsatile ambientlight, the photodetector being connected to the input of means 28 suchas a comparator which triggers the lamp 4 and the modulating means 10when the ambient light is at a minimum intensity.

The source 26 and pulse generator 30 are common to both the circuit forpowering the lamp 4 and the circuit for switching the modulating means10 and synchronise the flashing of the lamp 4 and the switching of themodulating means 10. The circuit for the lamp 4 further comprises apulse delay unit 32 and the flash lamp drive electronics 34, e.g. astandard drive unit for a strobe lamp. The circuit to switch themodulating means further comprises a pulse delay unit 36, a drivewaveform generator 38 (e.g. an arbitrary waveform generator manufacturedby Wavetech Ltd.) and an amplifier 40 to apply a potential difference,via the electrodes 14, 16 to the sheet 12. The pulse delay units 32, 36so affect the pulses applied to flash the lamp 4 and switch themodulating means 10 that a zero or low potential difference is appliedto switch the modulating means 10 to the first mode, translucent, whenpower is supplied to flash the lamp 4 and a potential difference of e.g.60V rms is applied, allowing the modulating means 10 to switch to thesecond mode, transparent, when no power is supplied to the lamp 4.

The use of a photodetector as the source 26 enables the contrast betweenthe image and noise produced by ambient light to be further improved.The photodetector measures the ambient light, e.g. from a fluorescenttube or a discharge lamp and is able to detect times at which the lightoutput of the fluorescent tube is minimal. This information is passed toa comparator 28 which causes the pulse generator 30 to generate pulsessuch that the modulating means 10 is in the first mode, i.e. effectiveas a screen, when the light output of the fluorescent tube is minimal.Accordingly the ambient light reflected from the modulating means isfurther attenuated.

It is further envisaged that an improvement in contrast of the projectedimage can be produced in accordance with the present invention bysynchronizing the operation of the modulating means 10 between the firstand second modes with a variation in intensity of the ambient lightalone, so that there is more ambient light when the modulating means isin the second mode than when the modulating means is in the first mode.Such a variation in intensity is inherent when the source of the ambientlight is a fluorescent tube or discharge lamp run from an AC mainssupply. Alternatively, if the ambient light is continuous, e.g. daylightfrom a window, the window may be made from NCAP or dyed NCAP which isswitched to produce a minimum in the ambient light.

The method described hereinbefore can also be used with modulating meanshaving other transmissive characteristics in the first and second modes.The essential feature for this method is that, the modulating means iseffective as a reflective screen in the first mode and reflects lesslight in the second mode than in the first mode. Accordingly therequired visual signal, the image reflected from the modulating means inthe first mode and seen by an observer, is the integral effect offlashing images maximally directed to the eye of the observer whereasany ambient light incident on either the front or the back of themodulating means is attenuated during the intervals between the flasheswhen the modulating means is in the second mode.

A second embodiment showing a second method of enhancing the contrastbetween an image projected onto a screen and the noise produced byambient light is illustrated in FIG. 4. The apparatus used is similar tothat used in the first method and accordingly like parts are designatedby like reference numerals.

The second method differs from the first method in that it is for use ina back-projection system. Light to form an image is focussed onto aplane P₂ in which is situated means 50 for modulating light. Themodulating means 50 provides a defined surface and comprises acombination of dyed NCAP material with a translucent sheet and isoperable between a first mode and a second mode. In the first mode, themodulating means 50 is translucent and forms a scattering screen fromwhich light is scattered forward to produce an image which can be seenby an observer (indicated at 51) on the other side of the modulatingmeans 50. In the second mode, the modulating means 50 is opaque andabsorbing and accordingly light incident thereon is not scatteredforward or reflected to the observer 51. Thus, as with the first method,the flashing images are maximally directed to the eye of the observer 51i.e. when the modulating means 50 is in the first mode whereas anyambient light is not scattered to the observer 51 during intervalsbetween the flashes. Accordingly, the contrast between the projectedimage, i.e. the required visual signal and the noise produced byscattered ambient light is improved. An anti-reflection film 52 in frontof the modulating means 50 reduces reflection of ambient light incidenton the front surfaces of the modulating means 50.

As with the first method, it is envisaged that an improvement incontrast of a projected image can also be produced by synchronizing theoperation of the modulating means with a variation in intensity ofambient light produced from a second source of light alone, or inaddition to a variation in intensity of light from the projector.

Alternatively, in another embodiment (not shown), the modulating meansis transparent in the first mode and opaque and absorbing in the secondmode. An assembly comprising a Fresnel lens and a lenticular screen(commonly used in prior art back-projector systems) is provided betweenthe projector and the modulating means. The combination of themodulating means and the assembly distributes light incident thereonwhen the modulating means is in the first mode and prevents light frombeing distributed therefrom when the modulating means is in the secondmode.

A third method of enhancing the contrast between an image projected ontoa screen and the noise produced by ambient light is exemplified by anembodiment of the present invention shown in FIGS. 5 and 6. This methoduses an apparatus similar to that shown in FIGS. 1 and 2 and accordinglylike parts are designated by like reference numerals.

The third method is for use in a front projection system. Light to forman image is focussed onto a plane P₃ in which is situated a reflectivescreen 60 formed of a translucent material or of an opaque andreflective material, such as aluminium paint, glass beads or lenticles.The reflective screen 60 provides the defined surface. Means 61 formodulating light is provided between the projector 3 and the screen 60.The modulating means 60 is effective as an electro-optic shutter, beingoperable between a first mode in which it is transparent and a secondmode in which it is absorbing, i.e. non-transparent. In the first mode,the modulating means 61 allows light from the flashing lamp 4 to beincident on the reflective screen and form an image which can beobserved by an observer (indicated at 62) in front of the modulatingmeans 61. In the second mode, the modulating means is absorbing and soprevents ambient light incident on the front surface of the modulatingmeans 61 being reflected to the observer 62 while also preventingambient light reaching the back surface of the modulating means 61 frombeing transmitted towards the observer 62. Thus, the effect of lightincident on the defined surface of the modulating means is reduced.

As shown in FIG. 6, the modulating means comprises a sheet 63 ofplastics material contains microencapsulated droplets of a nematicliquid crystal material including a dissolved dichroic dye (dyed NCAPmanufactured, inter alia, by Taliq Corporation, Sunnyvale, Calif.,U.S.A.). Such a material goes from absorbing to transparent when apotential difference is applied between the two transparent electrodes14 and 16.

FIG. 5 also shows an anti-reflection film 22 in front of the modulatingmeans 61 to reduce reflection of ambient light incident on the frontsurfaces of the modulating means 60 and an opaque layer 64 of a materialwhich prevents ambient light from being incident on the back surface ofthe screen 60. Suitable materials for such a layer 64 include afront-silvered material and an absorbing black material. Thus, as withthe first and second methods, this third method improves the contrastbetween the projected image, i.e. the required visual signal, and thenoise produced by ambient light.

In a modification of the embodiment of FIGS. 5 and 6, a reflectivescreen is used which has six dichroic layers containing cholestericliquid crystal material, in the form of liquid crystal polymer layers orprinted layers of emulsified or dispersed cholesteric droplets. Becauseof the properties of the cholesteric material, the layers of thereflective screen can be arranged to reflect light of wavelengths innarrow spectral bands in the red, green and blue parts of the spectrum.(Two layers with opposite senses of pitch are used for each spectralband required so that both left-handed and right-handed senses ofcircular polerisation are reflected). Thus, when ambient light, such asdaylight or normal artifical light, which has a broad spectraldistribution is incident on the screen, the major part of the energycarried is not reflected but transmitted by the screen. Only arelatively small proportion of the ambient light is reflected. Such ascreen is used in conjunction with a projector with light sourcesemitting light of wavelengths in the narrow spectral bands reflected bythe screen. As the brightness of the projected image can be controlledfrom the projector whilst the effect of ambient light is reduced, theuse of this screen further enhances the contrast between the projectedimage and ambient light.

As with the first and second methods, it is envisaged that animprovement in contrast of a projected image can also be produced bysynchronizing the operation of the modulating means with a variation inintensity of ambient light produced from a second source of light alone,or in addition to a variation in intensity of light from the projector.

In a further modification of this method, the modulating means isprovided in the form of a pair of spectacles or goggles worn by theobserver.

The embodiments of the invention described hereinbefore have used asingle source of light to project an image onto a single modulatingmeans. For ease of manufacture or operation, the modulating means couldbe constructed as a sheet of NCAP material with a plurality ofelectrodes on either side, optionally each with its own electronic drivecircuit.

The resolution of the image produced can be increased by using aplurality of projectors to form a plurality of interleaved or adjacentand abutting images of matching average luminance on the modulatingmeans. FIG. 7 shows a source picture 70 which has been divided intosections to be projected as an image by an array of M×N, in this case4×3 projectors. Each projector projects a respective section of thesource picture onto the modulating means to form an image. The compositeimage 72 produced is a matrix of images with no gaps therebetween.

A first modification of the methods described hereinbefore uses such anarray of projectors to produce a composite image. This is produced on amodulating means which can be switched between a first and a second modeto effect a screen in the first mode. The light sources of theprojectors are flashed simultaneously and in synchronism with theswitching of the modulating means such that images are projected whenthe modulating means is in the first mode.

In a second modification, as shown in FIG. 8, a source picture 76 isdivided into sections 76aa, 76ab, . . . 76mn to be projected to producean image by an array of M×N projectors. The screen 78 on which the imageis to be projected is formed as an array of M×N modulating means 78aa,78ab, . . . 78mn. Each modulating means (designated generally 78xy) isassociated with a respective projector and flashing light source suchthat the modulating means 78xy is in the first mode to effect a screenwhen the light source of the respective projector is on, projecting animage.

The modulating means 78xy can be switched independently of one another.In this way, the transmissive characteristics of a particular modulatingmeans can be adjusted to match the luminance of the image produced bythe respective projector. Alternatively, or in addition to this, themodulating means 78xy can be switched sequentially, with the flashing ofthe light source of the respective projector. This reduces flicker andspreads the power drain of the flashes more uniformly over time.

FIG. 8 shows a modulating means 78xy for each section of the sourcepicture 76. Alternatively, the screen on which the image is to beprojected can comprise a plurality of modulating means aligned to formrows or columns in the screen.

FIG. 9 illustrates a further modification of the methods which can beused to project a scanned two-or three-dimensional modulated lightpattern originating from a light source and represented by a matrix ofpixels to form a projected pattern or image. Such a pattern can beproduced using a scanned laser, a colour video projector or a cathoderay tube (CRT) based video projector (not shown). In a CRT based videoprojector, for example, (such as a beam-index type of colour CRT) anelectron beam is rapidly moved to form a raster (large number of scannedparallel lines on a display face). The display face is covered by one ormore phosphors which are excited to emit light by the electron beam.This light pattern which can be represented by a matrix of pixels isprojected onto a defined surface provided by a modulating means 80.

As shown in FIG. 10, the modulating means 80 comprises a sheet 82 ofNCAP material which goes from translucent to transparent when apotential difference is applied thereacross. One one side of the NCAPsheet is a plurality of horizontal strip indium tin oxide electrodes 86.Accordingly, the modulating means 80 is divided into a plurality ofhorizontal strip modulating means 88, each associated with a pluralityof pixels, which can be operated between a first mode and a second modeindependently of one another.

The operation of the strip modulating means 88 and the movement of theraster scan, which causes each pixel on the display face of theprojector to act as a flashing source of light, are synchronised. Thearrangement is such that (a) a respective modulating means 88 is in thefirst mode when its associated plurality of pixels are being scanned and(b) a respective plurality of pixels are not being scanned when itsassociated modulating means 88 is in the second mode.

As a pixel emits light, this is projected and incident on a particularstrip modulating means 88 which is in the first mode, i.e. translucent,to form a reflective screen. The switching of each strip modulatingmeans 88 is effected by respective drive electronics 90 connected torespective strip electrodes 86. This is controlled by a drive waveformgenerator and selector 92 which has as inputs a synchronising signal 94from the laser or CRT projector (e.g. via an infra-red link) andoptionally a size/distance control 95 which can be varied depending onthe separation of the modulating means 80 from the projector. Thesize/distance control 95 can be a manual control or an automaticcontrol, using an ultrasonic or infra-red rangefinder, which adjusts thetiming of the switching of different modulating means 88 to compensatefor different sizes of pictures obtained at different separations of themodulating means 80 and projector. Alternatively, the projector has azoom facility so that the same picture size can be obtained at differentseparations. An infra-red link may also be used to carry an audio signalfrom the projector to speakers at the screen.

At least a majority of those strip modulating means 88 on which lightfrom the CRT projector is not incident are in the second mode and sotransparent. Accordingly no light can be reflected from those stripmodulating means 88 to an observer on the same side of the modulatingmeans 80 as the projector (i.e. front projection). As with embodiment 1described hereinbefore, in a preferred embodiment, a layer of materialbehind the modulating means 80 absorbs ambient light incident on thefront surface of the modulating means 80 and also prevents ambient lightfrom behind the modulating means being observed by the observer throughthose strip modulating means that are transparent. In this way, themajority of the area of the modulating means 80 not being utilised bylight from the CRT projector is not effective to reflect any light backto the observer. Thus the noise produced by ambient light is reduced andthe contrast between the required visual signal, i.e. the projectedimage, and this noise is improved.

This modification has been described with reference to a modulatingmeans similar to that of embodiment 1. It is apparent that thismodification can also be applied to the other embodiments describedhereinbefore.

The method of the present invention is also beneficial to the productionof visual signals other than projected images as described hereinafter.

The second method can be used to produce a required visual signal whichis the appearance of transparent or translucent matter on a layer e.g.information on a backlit advertising hoarding, LCD or a road sign asshown in FIG. 11. The apparatus is similar to that of FIG. 4 andaccordingly like parts are designated by like reference numerals. Theinformation is provided on the defined surface of the screen 60. In thefirst mode, the modulating means 50 allows light from the flashing lamp4 diffused by a translucent layer 102 to be incident on the screen 100to illuminate the information. In the second mode, the modulating meansis absorbing and so prevents ambient light incident on the front andback surfaces of the modulating means being respectively reflected toand transmitted towards the observer. Thus the contrast between theilluminated information, i.e. the required visual signal, and the noiseproduced by any ambient light is improved.

A similar effect can also be produced using the third method toilluminate a screen from the front, particularly for the case where itis required to change the colour of the illuminating light.

A related application for embodiment 1 is the production of a visualsignal comprising an image projected onto printed information such as amap. At present, the production of such a signal is affected by therelative reflectivities of the inks used and the material on which theinks are printed. The method of the invention provides a modulatingmeans having a first mode which is either translucent or opaque andreflective and a second mode which is transparent. This is overlaid onan opaque material carrying the required printed information.

The method is shown schematically in FIG. 12. FIG. 12a shows themodulating means 106 in the first mode and so effective as a screen toprovide a defined surface on which an image 107 is projected. FIG. 12bshows the modulating means in the second mode which is transparent andaccordingly the printed matter 108 on the opaque material 109 is visibleto the observer. As the modulating means 108 is switched between thefirst and second modes, the projected image 107 and the printed mattercan be equally visible and so the resulting image seen by the observeris the combination shown in FIG. 12c. The reflectivity of the opaquescreen and the printed matter 108 does not affect the appearance of theprojected image 107 as this is projected not directly onto the opaquescreen 109 but onto the screen of the modulating means in the firstmode.

Another application of the invention can be considered generally underthe concept of `one-way windows`.

The concept is illustrated schematically in FIG. 13. A partition 110between a first section 112 and a second section 114 comprises, at leastin part, a modulating means 116. The modulating means 116 comprises anNCAP material and is switchable between a transparent mode and anon-transparent, e.g. translucent, mode. A first source of light 118 inthe first section 112 illuminates the first section and a second sourceof light 118 in the second section 114 flashes at a frequencysufficiently high to eliminate flicker.

The operation of the modulating means 116 and the flashing of the secondsource 118 are synchronised such that the second source 118 is off whenthe modulating means is the transparent mode, and on when the modulatingmeans is in the non-transparent mode. Accordingly, an observer 120 inthe second section 114 is able to observe the appearance of the firstsection 112 when the modulating means is transparent while he remains inthe illuminated environment of the second section 114. A person 122 inthe second section, however sees a translucent, i.e. frosted window whenthe modulating means 116 is in the non-transparent mode and the secondsource 118 is on but sees darkness when the modulating means 116 is inthe transparent mode. Accordingly, the integral effect seen by theperson 122 is a frosted window.

The effect of such a one-way window can be improved by providing ahalf-silvered mirror in addition to the modulating means. Alternatively,or in addition, the modulating means in the non-transparent mode can beabsorbing. The combination of a dyed NCAP sheet and an ordinary NCAPsheet can be used--the two sheets being transparent at the same time.

Such a one-way window can be used in many areas. One area is forsecurity when a security guard wishes to see into a room without beingobserved. Another area is motor vehicles at night in which thepassengers wish to have sufficient light to see but the driver wishes tobe able to see through a darkened passenger area so as to be able to seeout of the rear window--the modulating means would be opaque andabsorbing in the non-transparent mode and placed between the driver andthe passenger area. Similarly, such a modulating means used in theoutside windows with a flashing source of light in the motor vehiclewould prevent light from inside the motor vehicle distracting drivers ofother motor vehicles.

Another variation of this concept is shown in FIG. 14. A corridor 130 isdefined by a first partition 132a and a second partition 132b, eachcomprising, at least in part, a respective modulating means 134a, 134b.The respective sections 136a, 136b on the other side of the partitions132a, 132b are each illuminated by a source of light 138a, 138b flashingat a sufficiently high frequency to eliminate flicker. Each modulatingmeans 134a, 134b is operable between a transparent mode and anon-transparent mode.

The switching of each modulating means 134a, 134b is synchronised withthe flashing of the respective source of light 138a, 138b such that thelight is on when the modulating means 134a, 134b is transparent and offwhen the modulating means is non-transparent. Accordingly, each section136a, 136b appears to be continuously illuminated. However, the flashingof the sources of light 138a, 138b are in antiphase with each other, sothat the modulating means 134a of the first partition 132a isnon-transparent when the modulating means 134b of the second partition132b is transparent and vice versa. Thus, an observer 140, e.g. a nurse,in the corridor 130 is able to observe the appearance of the twosections 136a, 136b from the corridor. However, the privacy of people inthe two sections 136a, 136b from each other is preserved as the twomodulating means 134a, 134b are not transparent simultaneously.

The invention covers a wide range of methods and concepts. Otherembodiments within the scope of the present invention will be apparentto those skilled in the art.

In particular, it is envisaged the three methods described for enhancingthe contrast between an image projected onto a defined surface such as ascreen and the noise produced by ambient light scattered from thatdefined surface can be modified to produce a three dimensional image,e.g. a hologram, in a defined region.

I claim:
 1. A method of projecting a scanned two- or three-dimensionalmodulated light pattern originating from a light source and representedby a matrix of pixels, the method comprising the steps of:projectingsaid pattern onto a defined surface distant from the light source;operating a plurality of means for modulating light between at least afirst mode and a second mode, each of the modulating means beingassociated with a plurality of pixels; wherein the scanning of thematrix of pixels and the operation of said plurality of modulating meansare synchronised so that (a) a respective said modulating means is insaid first mode when its associated plurality of pixels are beingscanned and (b) a respective plurality of pixels are not being scannedwhen its associated said modulating means is in said second mode; andeach of said modulating means in said first mode allows light to bedistributed from said defined surface and each of said modulating meansin said second mode allows the effect produced by any light incident onsaid defined surface to be reduced.
 2. An apparatus for projecting ascanned two- or three-dimensional modulated light pattern originatingfrom a light source and represented by a matrix of pixels, the apparatuscomprising a light source;means for projecting a scanned two- orthree-dimensional modulated light pattern from said light source onto adefined surface distant from the light source; a plurality of means formodulating light having at least a first mode and a second mode, each ofthe modulating means being associated with a plurality of pixels; meansfor operating each of said modulating means between said first and saidsecond mode; and means for synchronizing the scanning of the matrix ofpixels and the operation of said plurality of modulating means so that,in use, (a) a respective said modulating means is in said first modewhen its associated plurality of pixels are being scanned and (b) arespective plurality of pixels are not being scanned when its associatedsaid modulating means is in said second mode; wherein each of saidmodulating means in said first mode allows light to be distributed fromsaid defined surface and each of said modulating means in said secondmode allows the effect produced by any light incident on said definedsurface to be reduced.
 3. An apparatus according to claim 2 wherein eachof said modulating means in said first mode forms a scattering screen toprovide said defined surface.
 4. An apparatus according to claim 3wherein said modulating means in said first mode forms a reflectivescreen.
 5. An apparatus according to claim 4 further comprising a layeron the other side of said plurality of modulating means from the matrixof pixels.
 6. An apparatus according to claim 5 wherein the layer isreflective when viewed from one direction and absorbing when viewed fromthe other direction.
 7. An apparatus according to claim 6 wherein thelayer comprises a louvred panel and a reflective layer.
 8. An apparatusaccording to claim 2 further comprising a screen for distributing lightto provide said defined surface, each of said modulating means formingan optical shutter, being transmissive in said first mode andnon-transmissive in said second mode.
 9. An apparatus according to claim2 wherein said modulating means is a layer containing a liquid crystalmaterial.
 10. An apparatus according to claim 9 wherein said modulatingmeans comprises a polymer film containing microdroplets of the liquidcrystal material.
 11. A method of projecting a scanned two- orthree-dimensional modulated light pattern originating from a lightsource and represented by a matrix of pixels, the method comprising thesteps of:projecting said pattern into a defined region distant from thelight source; operating a plurality of means for modulating lightbetween at least a first mode and a second mode, each of the modulatingmeans being associated with a plurality of pixels; wherein the scanningof the matrix of pixels and the operation of said plurality ofmodulating means are synchronised so that (a) a respective saidmodulating means is in said first mode when its associated plurality ofpixels are being scanned and (b) a respective plurality of pixels arenot being scanned when its associated said modulating means is in saidsecond mode; and each of said modulating means in said first mode allowslight to be distributed from said defined region and each of saidmodulating means in said second mode allows the effect produced by anylight incident on said defined region to be reduced.
 12. An apparatusfor projecting a scanned two- or three-dimensional modulated lightpattern originating from a light source and represented by a matrix ofpixels, the apparatus comprising a light source;means for projecting ascanned two- or three-dimensional modulated light pattern from saidlight source into a defined region distant from the light source; aplurality of means for modulating light having at least a first mode anda second mode, each of the modulating means being associated with aplurality of pixels; means for operating each of said modulating meansbetween said first and said second mode; and means for synchronizing thescanning of the matrix of pixels and the operation of said plurality ofmodulating means so that, in use, (a) a respective said modulating meansis in said first mode when its associated plurality of pixels are beingscanned and (b) a respective plurality of pixels are not being scannedwhen its associated said modulating means is in said second mode;wherein each of said modulating means in said first mode allows light tobe distributed from said defined region and each of said modulatingmeans in said second mode allows the effect produced by any lightincident on said defined region to be reduced.
 13. An apparatusaccording to claim 12 wherein each of said modulating means in saidfirst mode forms a scattering screen to provide said defined region. 14.An apparatus according to claim 13 wherein said modulating means in saidfirst mode forms a reflective screen.
 15. An apparatus according toclaim 14 further comprising a layer on the other side of said pluralityof modulating means from the matrix of pixels.
 16. An apparatusaccording to claim 15 wherein the layer is reflective when viewed fromone direction and absorbing when viewed from the other direction.
 17. Anapparatus according to claim 16 wherein the layer comprises a louvredpanel and a reflective layer.
 18. An apparatus according to claim 12further comprising a screen for distributing light to provide saiddefined region, each of said modulating means forming an opticalshutter, being transmissive in said first mode and non-transmissive insaid second mode.
 19. An apparatus according to claim 12 wherein saidmodulating means is a layer containing a liquid crystal material.
 20. Anapparatus according to claim 18 wherein said modulating means is a layercontaining a liquid crystal material.
 21. An apparatus according toclaim 19 wherein said modulating means comprises a polymer filmcontaining microdroplets of the liquid crystal material.
 22. Anapparatus according to claim 20 wherein said modulating means comprisesa polymer film containing microdroplets of the liquid crystal material.